1. Field of the Invention
This invention relates to flux gate electronic compass systems for providing dead reckoning navigation and more particularly to means for periodically compensating for local disturbance fields tending to affect the accuracy of the magnetic heading.
2. Description of the Prior Art
The simplicity, efficiency, small size, and reliability of the magnetic compass has made it an ever present and invaluable aid to navigation since its inception. With the advent of electronic compasses, the potential of the magnetic heading sensing systems has increased since such techniques remove the need for human interpretation of the raw heading information thereby allowing faster and more accurate use of this data.
The limiting factor on the accuracy of a magnetic heading sensor frequently lies in the anomalies in the magnetic field that is sensed. Often the anomalies caused by magnetic fields on the navigating vehicle itself are principal sources of error. Elaborate methods of measurement, degaussing and compensation have been devised to correct inaccuracies caused by this type of perturbation. Where errors cannot be corrected, tabulations of deviations are often made to be used to correct the compass reading to the true magnetic heading. The principal difficulty with such correction measures is that these perturbations can change with time and use of the vehicle.
The magnetic properties of metal structures depend on the type and history of the material. The orientation of the material when it is formed and shaped often accounts for its characteristics. A violent forming operation such as heating, forging, welding, riveting, etc. can influence the magnetic domains of the material, tending to align them with the local external field at the time. If these effects tend to remain over long periods of time, with the material retaining a magnetism, the material is called magnetically "hard". If any magnetism present in an external field tends to disappear upon removal of the field, the material is designated "soft".
Even permanently magnetized hard materials tend to vary with their history, however. Ships, for example when struck by lightening when undergoing extensive refitting and material changes when sitting in a port with a fixed heading for extensive periods or even when holding a fixed course for long durations, often experience changes in their magnetic characteristics, requiring a calibration of their compasses. Less data is available for land vehicle navigation systems but there is no reason to suppose that they are any different. The constant shock and vibration of road vehicles, particularly of the track variety, such as tanks and armored personnel carriers, may well affect the long term magnetic characteristics of the vehicle disturbances. If frequent recalibration is necessary and if the recalibration procedures are elaborate requiring extensive test equipment or special test areas, this can provide a serious drawback for the effectiveness of the compass system.